The present invention relates to an apparatus for bracing a plurality of vertebrae of the human spine.
Known supporting means operate with so-called lamina hooks which are disposed on a threaded rod. A compressing means is disclosed in British patent 2 131 300. A distracting means is disclosed in U.S. Pat. No. 4,382,438. The threaded rod bridges a plurality of vertebras and is not suited to act on vertebras which are disposed between the lamina hooks. Before using the supporting means a distraction or, respectively, compression has to be provided by means of an additional device.
German 90 06 646 U1 discloses an apparatus in which a lamina hook is replaced by a clamp-shaped holding member having a pair of legs of which one can be bent with respect to the other. Thereby the giapophysis of the vertebras may be used as anchoring points for the compressing and distracting apparatus.
German 88 02 112 U1 teaches a supporting device for the human spine, according to which the so-called pedicle screws are screwed in the pedicle body of the vertebras. The pedicle screws cooperate with tensioning means which bridge across one or more vertebrae to introduce forces between the vertebrae. The known device affords a primary stabilization of the vertebrae with respect to all degrees of freedom. However, when a number of vertebrae of a non-traumatic spine for example, are to be repositioned, separate means necessary to perform the reposition before the known supporting device may be effectively used.
WO 91/01691 discloses an apparatus for bracing vertebras of the human spine, comprising pedicle screws having slotted heads to receive a rod. The legs of the slotted screw heads include an outer thread on which a nut is screwed which contacts the rod to fix a predetermined position.
EP 0 443 892 discloses a similar device comprising a pedicle screw, the slotted head thereof including internal threaded portions for receiving a fixing screw which is brought into engagement with a serrated or similarly roughened rod to fix the relative position of the rod and the individual pedicle screw. A ring disposed around the head of a pedicle screw is provided so that the legs of the screw head do not spread apart while fixing the rod, as otherwise the engagement between the fixing screw and the screw head may be lost. A similar apparatus is disclosed in WO 90/09156.
Again, the devices last mentioned require separate means for repositioning, thus being suited to maintain a repositioned condition, but not suited to perform repositioning.
It is an object of the present invention to provide an apparatus which is suited to reposition the vertebrae of the human spine as well as to support the vertebrae in the repositioned position thereafter.
The objects are solved by the apparatus of the invention.
According to the invention, the apparatus uses a threaded rod (i.e., a distracting rod) in combination with lamina hooks. The thread of the rod, however, is not only used to fix the pedicle screws, but further is used to reposition the vertebrae in that an adjusting nut sitting on the threaded rod is turned with respect to the head of the pedicle screw and thus the vertebra has obtained the desired position. By means of the apparatus according to the invention a spine portion is not only distracted or compressed, but individual vertebrae can be effectively repositioned with respect to each other. To this end the threaded rod is designed to be relatively stiff and has a diameter between 7 to 8 mm, for example. On the other hand the rod must be bent to be implanted close to the spine along a bent spine portion. This is facilitated by the design of the pedicle screw heads having slots for receiving the threaded rod. Whereas the screw in the pedicle screw head according to EP 0 443 892, for example, axially fixes the rod, the securing screw of the present invention is merely used to prevent a deflection of the rod out of the receiving slot.
After resetting, the adjusting nut must be fixed on the threaded rod. This can be obtained by means of a suitable counter-nut. Still further, it is possible, to provide the adjusting nut and the front faces of the pedicle screw head with a rotary safety means in form of a toothing or another irregularity cooperating in a clamping fashion. In both cases the nuts are merely fixed by a frictional force. According to a further embodiment of the invention, however, the head of the pedicle screw has a width smaller than the diameter of the securing screw, whereas at least one front face of the adjusting nut has a recess cooperating with the securing screw. Preferably, the adjusting nut includes a plurality of peripherally spaced indentations, wherein the final rotary position of the nut is such that the securing screw cooperates with the indentation. This affords a positive locking of the adjusting nut resulting in a precise rotary locking which cannot be loosened.
The pedicle screws must take up relatively large forces. There is the danger that a pedicle screw breaks out of the bone, primarily when the available bony substance does not provide a sufficiently rigid seat in the vertebra. According to an embodiment of the invention, a mounting strap is attached to the shaft of the pedicle screw laterally extending therefrom, which strap includes an opening for receiving a spongiose screw. The strap has, for example, a pair of openings, wherein one opening receives the shaft of the pedicle screw, while the other opening disposed at the other end of the strap, for example, receives a spongiose screw which is screwed into the vertebra. In this manner, the pedicle screw is laterally stabilized and can receive substantial forces. According to an alternate embodiment of the invention, a mounting strap may be provided to the shaft of the pedicle screw extending therefrom, which strap has a blade or the like to be mounted in the vertebra. The blade is preferably integral with the strap. The blade is beaten into the vertebra wherein the strap may additionally include a hole for receiving a spongiose screw which is screwed into the vertebra. In some cases, a vertebra is displaced with respect to the adjacent vertebra. When the pedicle screw is completely screwed in, it cannot be connected any more with the threaded rod. To accomplish a connection, the pedicle screw is partly screwed into the vertebra bone. According to an embodiment of the invention, the head of the pedicle screw is then rotatably mounted on the screw shaft so that by rotating the shaft the vertebra can be pulled up to the threaded rod for resetting. Preferably the shaft includes tool engaging faces adjacent the head to rotate the shaft of the screw in the desired manner.
In case of very specially displaced individual vertebrae, not even the features referred to above are sufficient. According to a further embodiment of the invention, the upper end of the pedicle screw shaft is ball-shaped, while the pedicle screw head is defined by a ball-engaging cage including a slot for the rod. The cage can be arbitrarily positioned with respect to the shaft of the screw, but can exert a tensioning force to the screw shaft when being screwed in the vertebra. According to both embodiments just referred to, the threaded rod is inserted through a slot of the pedicle screw head, wherein a securing screw referred to several times prevents the rod from sliding out of the slot. Alternatively a closed passage may be provided in the head of the pedicle screw as it is known per se.
Instead of or in addition to the pedicle screw the apparatus of the present invention provides a hook cooperating with a lamina of a vertebra. Those lamina hooks are generally known. According to the invention, however, the lamina hook is provided with a slotted receiving portion to insert the threaded rod. The slot has threaded portions again to secure the rod in the receiving slot.